Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture in which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or "UUT." Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high-speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired test fixture" in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wire test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire-wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low-volume production boards where larger and more complex and expensive electronic test analyzers are not practical.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so-called "dedicated" test fixtures, also known as a "grid-type fixture," in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to interface pins arranged in a grid pattern in a receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures.
A typical dedicated or grid fixture contains test electronics with a huge number of switches connecting test probes in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 40,000 switches are used. When testing a bare board on such a tester, a translator fixture supports translator pins that communicate between a grid pattern of test probes in a grid base and an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins" are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base.
Translator fixtures can be constructed and assembled with a plurality of translator plates made from a plastic material such as Lexan. The translator plates are stacked in the fixture between corresponding sets of spacers aligned with one another vertically to form "stand-offs" spaced apart around the periphery of the fixture. The spacers hold the translator plates in a fixed position spaced apart vertically from one another and reasonably parallel to each other. The translator plates at each level of the fixture have pre-drilled patterns of alignment holes that control the position of each tilt pin in the translator fixture.
Several problems are associated with these types of test fixtures when the test points on the printed circuit board are positioned very closely together and are very thin. Individual test points are commonly referred to as test pads, and a group of test pads are commonly known as a test pack. When the tilt pins contact very thin test pads, the pads can be crushed or bent by the tilt pins. Depending upon the degree of damage to the test pads, and how closely they are positioned, individual pads can be permanently shorted together during testing.
A second problem occurring with these types of test fixtures is the difficulty in achieving accurate test results for a test pack when the pads are very closely spaced. It becomes very difficult to direct a tilt pin to each pad within the pack when the pads are so closely spaced. Slight misalignments of test pins can affect the test results, reducing test accuracy.
A third problem is encountered for packs having a grid density of pads which is greater than the grid density of the test probes, such as when the test pack is formed as a ball grid array (BGA) or a quad flat pack (QFP). In such instances there are not enough translation pins available for testing each test pad and thorough testing of the pack is not possible.
To address these problems a printed circuit board test fixture capable of accurately and safely testing circuit boards having small scale test packs was developed which included a pneumatically actuated shorting plate positioned in the fixture corresponding to the location on the printed circuit board where a group of very closely spaced test points were to be tested. A hole was cut through the upper translator plates corresponding to the dimension of the shorting plate to allow the shorting plate to engage the unit under test. A layer of compliant conductive media was positioned over the upper surface of the shorting plate for electrical connection to the test points. The shorting plate included a snap fitting for attachment to an air cylinder extending downwardly through the layers of translator plates. The air cylinder was attached at the bottom of the fixture by a base plug which snaps into a base receptacle rigidly secured to a lower translator plate of the fixture.
During testing of the unit under test, the air cylinder was energized, raising the shorting plate into contact with the test pack, effectively shorting them together for testing without bending or damaging the test points.
A problem with this method is that since all the test sites are shorted together during testing it can not be determined whether one or more individual test sites within the pack are incorrectly shorted together.
An alternative method for testing densely spaced test packs is with a prober to touch each individual pad within the pack. This method is undesirable due to the extremely time consuming process.
Consequently a need exists for test equipment for testing printed circuit boards having small scale or densely spaced test sites, which accurately, safely and quickly produces the test results.